The present invention relates to a ball bearing used for rotatably, axially supporting a rotary member on a fixing portion and a method of assembling the same and, more particularly, to a ball bearing in which part of a rotary member serves as an inner ring and a method of assembling the same.
A ball bearing of this type is generally used for rotatably or pivotally, axially supporting a rotary member or rotating shaft in the holding hole of a holding member as a stationary member. As the ball bearing of this type, one in which an inner ring is integrally formed with part of the rotary member is conventionally known.
This ball bearing is assembled by fitting an outer ring on an inner ring portion formed in part of the rotary member, incorporating a plurality of balls between the inner and outer rings, arranging the balls in an annular gap between the inner and outer rings equidistantly at predetermined distances in the circumferential direction, and thereafter mounting a holder for maintaining the gap of the balls.
In assembly of the ball bearing described above, when the rotary member is a shaft member, an assembly jig which has a hole portion for holding the shaft member extending in it is used. More specifically, the outer ring is fitted on the inner ring portion of the rotary member which projects upward on the assembly jig. This outer ring is set on one side with respect to the inner ring so as to be eccentric. The gap between the inner and outer rings is partially increased. The plurality of balls are incorporated through this large gap portion. The outer ring is located to be concentric with the inner ring, so that the balls are interposed between the inner and outer rings. The balls that are set on one side are located at predetermined distances in the annular gap between the inner and outer rings so as to be equidistant from each other. Thereafter, the holder having pawl portions for maintaining the gap of the balls is mounted.
Such a ball bearing is used in, e.g., a rack-pinion type power steering apparatus, to rotatably hold a pinion shaft, integrally formed with a sleeve serving as the outer valve member of a rotary type channel selector valve (rotary valve), on a steering body serving as a stationary portion. It is proposed to rotatably, axially support the pinion shaft on the steering body by using a ball bearing in which the end portion of the pinion shaft described above serves as the inner ring. With this arrangement, a ball bearing that requires a space in the direction of diameter can be incorporated in a steering body, which has been demanded to be made compact, comparatively easily.
In this power steering apparatus, when the apparatus operates, a large load sometimes acts on the pinion shaft integrally formed with the sleeve serving as the outer valve member constituting the rotary type channel selector valve, to cause deformation. Then, decentering and offset contact occur between the sleeve and the rotor serving as the inner valve member, and a smooth valve action cannot accordingly be obtained. For this reason, a ball bearing which is expected to perform reliable axial support is generally used.
In the conventional ball bearing described above, when fitting the outer ring on the outer circumferential side of the rotary member serving as the inner ring and incorporating and interposing a plurality of balls between the inner and outer rings, it is difficult to interpose the balls at predetermined portions between the inner and outer rings.
In the rotary type channel selector valve of the power steering apparatus described above, the pinion shaft having the sleeve serving as the outer valve member is an elongated shaft member, and a large-diameter portion, a small-diameter portion, grooves, and gears are formed on its outer circumferential portion. To assemble such a ball bearing in which the shaft end portion of the pinion shaft serves as the inner ring, the following operation is required. More specifically, the pinion shaft is held by the assembly jig substantially vertically. The outer ring is fitted on the inner ring portion. The outer ring is set on one side to increase the gap between the inner and outer rings. The plurality of balls are incorporated through this large gap portion, and the outer ring is moved to be located concentric with the inner ring. Then, the balls are moved to predetermined equidistant positions. This assembly, however, has a problem in that it requires skills.
This problem will be described in detail. When assembling the ball bearing described above, if the balls are incorporated through the gap between the inner ring and the outer ring which is set on one side, the balls cause positional errors more than necessary between the inner and outer rings and sometimes drop accordingly. More specifically, when the outer ring is set on one side as described above and the balls are incorporated in the gap between the inner and outer rings, the balls, especially the centers of the balls, shift below the edges of the groove-side edges of ball holding grooves respectively formed in the outer circumferential surface of the inner ring and the inner circumferential surface of the outer ring. Then, the outer ring cannot be moved to be concentric with the inner ring. Even if the outer ring is moved, the balls cannot be held in the ball holding grooves between the inner and outer rings.
Therefore, in assembly of the ball bearing in which part of the rotary member serves as the inner ring, as described above, assembly of incorporating the balls between the inner and outer rings and holding the balls in the ball holding grooves requires the greatest care, is cumbersome, and has poor operability, posing many problems in achieving automatic assembly which has been sought for in recent years.
The balls are shifted below as described above because the amount of eccentricity, which is caused when the outer ring is set eccentrically with respect to the inner ring in order to incorporate the balls between the inner and outer rings, is larger than necessary. This is due to the following reason. To incorporate the balls between the inner and outer rings, a gap larger than the ball diameter is necessary. If the outer ring is to be simply set eccentric, the outer ring can be set eccentric until its inner circumferential surface comes into contact with the outer circumferential surface of the inner ring. Although depending on the sizes of the respective portions, the outer ring becomes largely eccentric to exceed the ball diameter, and the balls undesirably pass through the gap between the inner and outer rings.
The problem of operability during assembly appears conspicuously when the balls cannot but be incorporated only in one direction. For example, assume that the pinion shaft has a complicated shape, like the pinion shaft serving also as the outer valve member (sleeve) of the rotary type channel selector valve in a rack-pinion type power steering apparatus, and that a flange portion to mount a seal ring on it projects close to a portion serving as the inner ring. In this case, unlike in assembly of an ordinary ball bearing, the balls cannot but be incorporated only in one direction. The above problem appears conspicuously in such a case.